Our experiments focus on the complex cellular interactions underlying early development of the mammalian embryos, and in particular signals provided by members of the TGFbeta superfamily. We have previously shown that nodal activity is required for primitive streak formation and anterior patterning during gastrulation, and at later stages for the establishment of the L/R body axis. We have also analyzed functions contributed by Smad proteins, intracellular effectors of TGFbeta signaling, and described an essential role for Smad2 during establishment of the A-P axis. Experiments outlined in this renewal application aim to characterize contribution(s) by the nodal signaling pathway during morphogenesis of the mouse organizer. We will use gene targeting to selectively delete the node-specific enhancer, and examine the phenotype of the resulting embryos. Using a novel in vivo cell marking system, we will investigate contributions made by nodal, HNF3beta and brachyury (T) during cell fate specification and early mesodermal patterning. We will describe the fate of cells derived from the mouse organizer, and how the expression of these genes may contribute to the formation of possibly distinct cellular descendants of this complex tissue. To further describe nodal activities during establishment of the L/R body axis, a gene targeting approach will be used to manipulate asymmetric nodal expression. Finally, experiments will be performed to describe the unique functions contributed by Smad family members, the effectors of TGFbeta signaling pathways, during embryonic development.